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Date: Fri, 26 Jan 2024 10:36:34 -0500
From: Johannes Weiner <hannes@cmpxchg.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Shakeel Butt <shakeelb@google.com>,
	Muchun Song <muchun.song@linux.dev>, cgroups@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kernel test robot <oliver.sang@intel.com>
Subject: Re: [PATCH] mm: memcg: optimize parent iteration in
 memcg_rstat_updated()
Message-ID: <20240126153634.GH1567330@cmpxchg.org>
References: <20240124100023.660032-1-yosryahmed@google.com>
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In-Reply-To: <20240124100023.660032-1-yosryahmed@google.com>

On Wed, Jan 24, 2024 at 10:00:22AM +0000, Yosry Ahmed wrote:
> In memcg_rstat_updated(), we iterate the memcg being updated and its
> parents to update memcg->vmstats_percpu->stats_updates in the fast path
> (i.e. no atomic updates). According to my math, this is 3 memory loads
> (and potentially 3 cache misses) per memcg:
> - Load the address of memcg->vmstats_percpu.
> - Load vmstats_percpu->stats_updates (based on some percpu calculation).
> - Load the address of the parent memcg.
> 
> Avoid most of the cache misses by caching a pointer from each struct
> memcg_vmstats_percpu to its parent on the corresponding CPU. In this
> case, for the first memcg we have 2 memory loads (same as above):
> - Load the address of memcg->vmstats_percpu.
> - Load vmstats_percpu->stats_updates (based on some percpu calculation).
> 
> Then for each additional memcg, we need a single load to get the
> parent's stats_updates directly. This reduces the number of loads from
> O(3N) to O(2+N) -- where N is the number of memcgs we need to iterate.
> 
> Additionally, stash a pointer to memcg->vmstats in each struct
> memcg_vmstats_percpu such that we can access the atomic counter that all
> CPUs fold into, memcg->vmstats->stats_updates.
> memcg_should_flush_stats() is changed to memcg_vmstats_needs_flush() to
> accept a struct memcg_vmstats pointer accordingly.
> 
> In struct memcg_vmstats_percpu, make sure both pointers together with
> stats_updates live on the same cacheline. Finally, update
> mem_cgroup_alloc() to take in a parent pointer and initialize the new
> cache pointers on each CPU. The percpu loop in mem_cgroup_alloc() may
> look concerning, but there are multiple similar loops in the cgroup
> creation path (e.g. cgroup_rstat_init()), most of which are hidden
> within alloc_percpu().
> 
> According to Oliver's testing [1], this fixes multiple 30-38%
> regressions in vm-scalability, will-it-scale-tlb_flush2, and
> will-it-scale-fallocate1. This comes at a cost of 2 more pointers per
> CPU (<2KB on a machine with 128 CPUs).
> 
> [1] https://lore.kernel.org/lkml/ZbDJsfsZt2ITyo61@xsang-OptiPlex-9020/
> 
> Fixes: 8d59d2214c23 ("mm: memcg: make stats flushing threshold per-memcg")
> Tested-by: kernel test robot <oliver.sang@intel.com>
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202401221624.cb53a8ca-oliver.sang@intel.com
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>

Nice!

Acked-by: Johannes Weiner <hannes@cmpxchg.org>